Sulfur-vulcanizable elastomers based on the condensation of thiodiethanol with one or more aliphatic diols, and containing a sulfur-vulcanizable double bond, are described by Aloia, U.S. Pat. No. 3,951,927; Chang et al, U.S. Pat. No. 3,985,708; Chang, U.S. Pat. No. 4,000,213; and Li et al, U.S. Pat. No. 4,028,305, which patents are incorporated herein by reference. These elastomers may be represented by the Formula (I): EQU H--OG--.sub.n OH (I)
wherein --OG-- comprises a copolymer of randomly alternating structural units selected from:
(A) structural units (II) and (III) EQU --OC.sub.2 H.sub.4 SC.sub.2 H.sub.4 -- (II) EQU --OR-- (III)
wherein R represents one or more radicals remaining on removal of two hydroxyl groups from (a) aliphatic saturated linear, branched chain or cyclic diols, or (b) aliphatic linear, branched chain or cyclic diols containing external unsaturation and having an allylic hydrogen atom; and
(B) structural units (II), above, (III), above, and (IV) EQU --OR'-- (IV)
wherein R' represents the radical remaining on removal of two hydroxyl groups from a diphenolic compound; said copolymers comprising structural units (A) and (B) being characterized in that:
(1) n is an integer sufficient to provide in said copolymer a molecular weight of at least about 8000; PA1 (2) the molar ratio of (II) to (III) when said copolymer comprises structural units (A), or the molar ratio of (II) to the total of (III) and (IV) when said copolymer comprises structural units (B), is not less than about 1:1; and PA1 (3) the polymers contain from about 1 to 10 mole percent of said diol (b), based on the total of all units (II), (III) and (IV) present in said copolymer.
Exemplary of such copolymers are those comprising at least 50-mole percent of thiodiethanol, 40-49 mole percent of one or more saturated aliphatic diols, e.g., diethylene glycol, and 1 to 10 mole percent of an unsaturated aliphatic diol, e.g., trimethylol propane monoallyl ether.
The thiodiethanol-based elastomers are readily vulcanized with sulfur in a non-zinc oxide containing formulation. The preferred vulcanization systems include a mixture of sulfur and a decahydropyrazine[2,3-b]pyrazine compound, as described by Maulding and Behrens in copending application, Ser. No. 017,655, filed about Mar. 5, 1979, now U.S. Pat. No. 4,202,963, and a mixture of sulfur and a polyethylene amine compound, as described by Behrens in copending application, Ser. No. 017,656, filed about Mar. 5, 1979, now U.S. Pat. No. 4,218,559 both of which are incorporated herein by reference thereto.
The elastomers, prepared as described, and vulcanized with sulfur, exhibit outstanding properties. However, the vulcanized elastomers are not as thermally stable as desired, i.e., they suffer loss of properties (tensile strength, elongation) on aging at elevated temperatures. One of the most serious disadvantages of the thermal instability is manifested by high compression-set properties.